Using marijuana daily for four years or longer may be related to certain changes in the brain, according to new research.

In the study, researchers used magnetic resonance imaging (MRI) to examine the brains of 48 adults who were chronic marijuana users, meaning they used the drug at least three times a day. Researchers also looked at 62 people who didn't use marijuana.

The investigators found that the people who had been smoking marijuana daily for at least four years had a smaller volume of gray matter in a region called the orbitofrontal cortex, which is commonly associated with addiction.

These users also showed greater connectivity between different parts of the brain, compared with nonusers. (Connectivity is a measure of how well information travels between different parts of the brain.)

"We found that there … not only is a change in structure, but there also tends to be a change reflected in the connectivity," said study author Francesca Filbey, an associate professor in the School of Behavioral and Brain Sciences at the University of Texas at Dallas.

The lost brain volume could explain the increased connectivity found in marijuana users' brains, Filbey told Live Science. The brain's connectivity may increase "to compensate for the loss in grey matter volume in that region," she said.

But it also possible that these differences in connectivity and the size of the brain region were present in the people in the study before they started using marijuana, she noted.

"All we can say is that we do see these" differences in people who use marijuana, Filbey said.

Still, there is reason to think marijuana did cause the differences. "We also saw that the younger you are when you start using marijuana regularly, the greater the changes in the brain," she said. Interestingly, the increased connectivity was not seen in the people who had been using marijuana for six to 10 years, she noted.

The differences in the brains of the marijuana users may have something to do with THC (tetrahydrocannabinol), which is the main psychoactive chemical in marijuana. THC affects cannabinoid receptors in the brain, which are involved in regulating appetite, memory and mood, and the orbitofrontal cortex has many of those cannabinoid receptors, Filbey said.

"If someone smokes marijuana, this area is bound to be affected by it," she said.

The researchers suspect that the changes in the brain occur to adapt to the THC in a person's system, she said.

It is difficult to say what consequences these changes may have for users, Filbey said, as it likely depends on the individual. However, previous research has found that the orbitofrontal cortex is very involved in the addiction process and in how people respond to marijuana, Findley said.

"In our other studies, we did find that those who had worse problems with marijuana [addiction] had a greater response in that area," as the area was very hypersensitive to marijuana use, she said.

The study was published Nov. 10 in the journal Proceedings of the National Academy of Sciences.